A magnetron microwave oscillator is composed of a cathode and a surrounding coaxial anode. The anode has a multiplicity of resonant cavities that interact with azimuthally circulating electron bunches emitted from the cathode to generate microwave radiation. The microwave frequency is principally determined by the dimensions and number of resonant cavities, the magnitude of an externally applied magnetic field and the voltage between the cathode and the anode.
A number of methods exist to tune the frequency of a magnetron. U.S. Pat. No. 3,671,801 to Masek discloses a tunable magnetron. A spring loaded plunger inserts a tuning rod into a resonating anode cavity. U.S. Pat. No. 5,182,493 to Robertson discloses a tunable rising sun magnetron. A tuning plate is extendable into a number of resonant cavities to change the dimensions of a number of adjacent cavities. Both the Masek and the Robertson patent are incorporated by reference herein in their entireties.
Both of the above techniques produce small perturbations in resonance and produce correspondingly small frequency changes. This benefit is at the expense of increased susceptibility to starting instabilities and arcing due to the tuning mechanism occupying a portion of the resonant cavity volume.
Coaxial magnetrons, as disclosed in U.S. Pat. No. 5,041,801 to Squibb, are tuned by moving a plunger in an externally coupled cavity. This eliminates the limitations of the conventionally tuned magnetrons, but the tunable range and output power are both still restricted by the requirement that the external cavity Q-value (a measurement of oscillation quality, measures the RF losses in the external cavity) must be high enough to efficiently store the microwave energy from the magnetron proper.
There remains, therefore, a need for a magnetron that is tunable over a wide range of output frequencies without a limitation imposed on the output power.